Portal 2 could well have been a by-the-numbers sequel. The fully-fledged follow-up to the cult downloadable smash increases its output two-fold, with two campaigns to play, another sidekick's quips to soak up, and the opportunity for double the players and, bafflingly, another set of portals as well. Except, it really isn't an ordinary follow-up: not only does it prove that the original's charms were by no means a fluke, but it far exceeds them. What's here is no only incredibly smart and brilliantly balanced in terms of puzzles and trials, but the narrative is equal parts hilarious, bleak and bizarre, and the way in which it's presented is second-to-none.

While the game's story follows on from the events of the original, it is self-contained enough so that newcomers can cleanly experience it. Waking up from a period of stasis, returning protagonist and test subject Chell bumps into a bumbling robotic sphere, Wheatley, and together the pair make their way through a now-ruined, dilapidated Aperture Science in a bid to escape. After accidentally reanimating GLaDOS - an evil AI overlord dedicated to science and experimentation - the duo must make their way past her attempts to restore the facility, which includes a whole new breed of mind-bending test chambers and deadly obstacles.

Although the original's laboratory setting had a unique and fitting sparkly-white aesthetic, Portal 2 offers a weathered and organic world that's far more appealing. Plants and moss have now seep through cracks, and many of the game's test chambers now bustle with life as GLaDOS reorganises the place as she sees fit, with broken walls shunting back into position and decayed tiles spinning around until they become new again. As well as a look at an aging Aperture and its inner workings, it also provides a fascinating insight into its past, and without delving into specifics, the way in which this is handled is incredibly clever.

In fact, the way in which the story is delivered is nothing short of sublime. It's a campaign that constantly pushes the player forward, feeding them a new puzzle mechanic here or mysterious sight or sound there, and pads out what would be ordinary excursions between test chambers or along humble walkways with reams of engaging dialog from its entertaining cast. GLaDOS is her same, raspy and endlessly entertaining self, and the casting of Wheatley (voiced by Stephen Merchant) is an inspired one. Between its script and smart array of environmental cues, the game does a tremendous job at fleshing out the bleak and bizarre world in which you're trapped.

After getting to grips with the Portal gun once more - the bread and butter of the game's puzzles, allowing you to bend space by creating two interconnected doorways - the game gently introduces you to all kinds of novel mechanics that allow players to experience areas in all new ways. There are aerial faith plates, which propel objects and players across the room at dazzling speed, as well as walkable bridges of light and funnels that suspend objects in mid-air. Both can be fed through portals and spread across vast expanses to connect new areas and objects together. Then there are more player-specific additions, with gels that allow surfaces to bounced and slid upon once coated. They're all fascinating toys by themselves, but the game manages to mix them together and put them alongside existing objects - such as those meekly-voiced turrets and lovable companion cubes - that feels natural but always presents a challenge.

While the game will throw you in some utterly baffling situations - such as vast rooms with a seemingly disconnected series of objects that somehow must be sandwiched together to progress - it always does so with a guiding hand, placing the most obvious starting point in clear view and then letting you run with it. It also does the opposite rather wonderfully, too, by presenting you a seemingly straightforward situation that's littered with red herrings. While it is very possible to get stuck - a help system of some sort would ultimately be beneficial - solutions are quite easily within reach with trial and error, and for those patient enough to solve it themselves, those eureka moments are something to be treasured.

The campaign isn't perfect, however: while the way in which it's delivered is fantastic, how the story actually plays out is ultimately a little unsatisfactory, especially the final third of the game. With the final boss vanquished and the credits rolling, you do wish there was an hour here or there to make the plot a little more well rounded. Structurally it also plays it very safe - the entire campaign is effectively three (albeit uniquely themed) series of test chambers, chopped up with the occasional escape scene or moment of gentle exploration. Although it's the perfect format for delivering puzzles - and again, is all strung together with magnificent pacing - it's a format that's a little too familiar to the original.

Elsewhere, the co-operative campaign offers entirely new content and a whole new series of challenges. With a second set of portals thrown into the mix, not only are puzzles specifically designed to require two players - nothing here is physically possible in the single-player campaign - but a second person certainly helps you work your head around what's in store. As such, it does genuinely feel like a unique addition rather than a separate story and new areas cobbled together like co-op campaigns in other games, but even in that respect it's commendable. GlaDOS constantly leads players through chambers and mysterious off-site locales on unexplained missions, and joyfully attempts to play them off one another with a spot of fourth-wall-breaking ribbery. It also provides users with a series of gestures and countdown timers to smartly smooth over any cracks in communication. While the connection to the campaign's story is weak at best (which is probably a sensible approach), the amount of genuinely fresh and well-implemented content makes it a very welcome part of game.

Portal 2 is one of those rare games where the usual negative points are vastly, vastly overshadowed by the positives. When the campaigns are finished (you'll easily clear both in a matter of evenings), is there actually much more to do besides the prospect of downloadable content? Not really. Is the story, as unfortunate as it is to admit, slightly underwhelming in places? It is. But ultimately, few games can offer such a wonderful universe to visit, a warm cast that makes you genuinely grin from ear to ear, and a series of trials so perfectly pitched that they are equally challenging and fair. Portal 2 may have a few sticking points, but what's there is so well implemented and consistent from start to finish that it's impossible not to give it anything less than full marks.